Cellulases are of considerable current interest for converting the cellulosic content of biomass to fermentable sugars for biofuels production. Several enzymes (exoglucanase, endoglucanase, and β-glucosidase) act in concert to hydrolyze cellulose to glucose. The filamentous fungus Trichoderma reesei is an industrially used cellulase producer because of its remarkable ability to secrete large quantities of cellulolytic enzymes (>50 g/L). The fungus produces at least seven extra-cellular cellulases that are required for the complete degradation of crystalline cellulose. These include two exoglucanases, called Cel7A and Cel6A (CBH I and CBH II), that represent 50% and 20% of the total cellulase content, respectively; and five endoglucanases, Cel7B, Cel5A, Cel12A, Cel61A, and Cel45A (EG1, EG2, EG3, EG4, and EG5) that represent 15%, 10%, 1%, <1% and <1% of the total cellulase content, respectively.
Lignocellulose hydrolysis using cellulases at high temperatures has many potential advantages such as higher solid loadings due to reduced viscosity, lower risk of microbial contamination, greater compatibility with high temperature pre-treatments, and faster rates of hydrolysis. However, T. reesei cellulases and other fungal enzymes have relatively low activity and stability at higher temperatures (Takashima, S et al., J Biotechnol 65(2-3):163-71, 1998). The ability to produce and secrete adequate quantities of cellulases is paramount for the economical production of biofuels from cellulosic biomass, and the expression and production of thermophilic cellulases from non-fungal sources has so far been severely limited (<100 mg/L).
Thus, a need exists in the art for thermostable fungal cellulases that can hydrolyze lignocellulose efficiently at higher temperatures.